Quantitative information on the thermodynamics of the conformational changes in hemoglobin are essential for a better understanding of the energetic of the hemoglobin system and the regulation of its oxygen affinity in vivo and in vitro. The objectives of this proposal are to measure the molecular weight distribution of partially ligated systems of hemoglobin, under conditions in which the liganded species dissociate in dimers while the unliganded specie is practically tetrameric. Also under study are the conformations of the unliganded derivatives of several mutant and preparative hemoglobins. In some of these systems it was demonstrated that ligand binding cooperativity could be restored without a simultaneous formation of a normal "deoxy" conformation. The correlation between exposure to the solvent of tyrosol residues and binding cooperativity will be investigated. The interaction of apohemoglobin with polyanions should clarify the relevance of tetrameric assembly and of interaction with polyacids to the stabilization of the tetrameric and helicoidal structure in the hemoglobin system. The electronic spectra of various derivatives of hemoglobin will be analyzed trying to correlate conformtional changes of the protein to changes in the electronic configuration of the heme.